1. Field of the Invention
The present invention relates to a gas laser electrode for exciting laser gas, a laser chamber employing such electrode, and a gas laser device.
2. Description of the Related Art
In a laser chamber of a typical gas laser device such as an excimer device, provided are primary ignition electrodes facing each other and arranged so as to sandwich an optical axis of a laser for exciting the laser gas filled inside the laser chamber and causing laser oscillation, and preionization electrodes for preionizing the space between the primary ignition electrodes so as to make the discharge of electricity between such electrodes easier. As the laser gas, for example, used may be a mixed gas of noble gas and halogen gas.
In this type of gas laser device, by the primary ignition electrodes being preionized with the preionization electrodes, laser gas is excited by the generation of electric discharge between the primary ignition electrodes, thereby causing laser oscillation. Moreover, as general knowledge, laser oscillation is stabilized when the electric discharge between the primary ignition electrodes is stable and, as a result, a stable laser output can be obtained thereby.
As examples of primary ignition electrodes to be employed in this type of gas laser device, known are those described in, for instance, Japanese Utility Model Application Laid-Open No. S61-1774764 (hereinafter referred to as “Document 1”), Japanese Patent Application Laid-Open No. S62-199078 (hereinafter referred to as “Document 2”), and Japanese Patent Application Laid-Open No. S63-227069 (hereinafter referred to as “Document 3”).
With the primary ignition electrode described in aforementioned Document 1, in order to obtain large laser output, an insulator is adhesively coated on the side face areas excluding the primary ignition portion in the primary ignition electrode in order to prevent the generation of electric discharge between the primary ignition electrode and the arc discharge electrode (corresponds to the spare ionization electrode) even if the spacing between such primary ignition electrode and arc discharge electrode is shortened.
Further, with the primary ignition electrode described in aforementioned Document 2, a halogenous corrosion resistant resin layer is coated on a laser tube or at least a part of an electric discharge material in order to avoid the encounter of defects such as strong ultraviolet rays in the vicinity of the primary ignition electrode, corrosion of laser tube walls and electric discharge materials due to considerable generation of ions and electrons, deterioration of the filled gas, and so on.
Moreover, with the primary ignition electrode described in aforementioned Document 3, an insulator is applied to the end of the primary ignition electrode, for example, the end phase thereof, in order to obtain a stable glow discharge at the plane face of the center portion of the primary ignition electrode and suppress the dielectric breakdown and arcing at the primary ignition electrode end.
Further, as primary ignition electrodes other than those described in the aforementioned documents, known is a primary ignition electrode wherein dielectric thin film is applied to the cathode surface in the primary ignition electrode structured from an anode and cathode. Here, by applying a dielectric thin film to the cathode surface, the deterioration of the anode (deformation of electrode) caused by the impact of electric discharge is reduced by utilizing the phenomenon of the decrease in breakdown voltage.
Nevertheless, with the primary ignition electrodes described in aforementioned Documents 1˜3, laser output characteristics would suddenly change and the maintenance of the initial characteristics becomes difficult a result of the deformation of the anode surface (deformation into a convex-concave shape) due to the impact of electric discharge and the transformation of the electrode due to halogenation of electrode materials caused by the erosion with halogen gas at the anode surface. In other words, it is not possible to obtain stable and desired laser output characteristics.
For example, with the primary ignition electrodes to be provided in the laser chamber employed in excimer laser devices such as a krypton fluorine (KrF) excimer laser or an argon fluorine (ArF) excimer laser, fluorine (F2) contained in the mixture of noble gas (krypton Kr, argon Ar) and halogen gas (fluorine F2) reacts with the discharging portion of the anode as a result of laser oscillation operation being repeated, and the anode is thereby halogenated (in this case, fluorinated). Simultaneously, the discharging portion of the anode deforms from a plane state to a convex-concave shape.
Thereby, problems would arise in that the electric discharge between the primary ignition electrodes becomes unstable, the output energy of the laser decreases and, as a result, desired laser output characteristics can not be obtained.
In order to overcome such problems, it is necessary to take measures such as raising the gas pressure in the laser chamber or raising the voltage to be applied between the primary ignition electrodes. Moreover, in particular cases, the deteriorated anode (or the primary ignition electrode) must be replaced, resulting in inefficient workability. Further, even if such deteriorated electrode is replaced with a new electrode, the aforementioned problems would similarly arise and, as a result, the cycle of replacing the electrode would occur frequently, resulting in increased maintenance costs.
Meanwhile, even with the primary ignition electrodes wherein a dielectric thin film is applied to the cathode surface thereof, as the discharging portion at which electricity is discharged between the anode and cathode is not coated, similar to the above, there is a problem in that stable laser output characteristics could not be obtained as a result of the deterioration of the anode (deformation of electrode) due to the impact of electric discharge, and the transformation of the electrode (anode) due to the halogenation (fluorination, for example) of the electrode materials.